This invention relates to methods and apparatus for optical data switching.
Fibre optic networks have traditionally been used for the circuit-switched transport of data, e.g. in telecoms networks. Data travelling between two nodes follows a dedicated circuit or path through the network, from start to finish. This is a very reliable way of transporting data, since there is no contention between data travelling along different paths. The end-to-end transport delay is low and predictable.
However, circuit switching is usually wasteful of resources, since bandwidth must be reserved for a particular path even if that path is empty of traffic for much of the time.
An alternative model for directing network traffic is packet switching, in which packets of data are given a destination address and are allowed to travel along a “best effort” route from start to destination, as determined by routers or packet switches along the way. Packets from the same start node may follow different routes to the same destination at different times. At intermediate nodes, or routers, packets may be queued in a buffer, until there is capacity for them to be sent on their next hop. Packets may be dropped if they cannot be routed after a period of time. There is thus no guarantee of timely delivery for packets. Packet will not necessarily arrive in the order in which they were sent.
Packet switching is flexible in its use of available bandwidth, typically resulting in a more efficient use of resources than circuit switching. However, it is not well suited to traffic that must be delivered within a maximum delay period and with negligible packet loss, such as video-conferencing data, live broadcast data and synchronisation signals.
The present inventor has previously contributed to the development of a hybrid approach that combines the flexibility of packet switching with the reliability of circuit switching. This is described in “A Packet-Switched Hybrid Optical Network with Service Guarantees” by Steinar Bjornstad et al., IEEE J. Sel. Areas Commun., Supplement on Optical Communications and Networking, vol. 24, no. 8, pp. 97-107, August 2006.
In the paper referred to above, two classes of data packet are defined: Guaranteed Service Traffic (GST) and Statistically-Multiplexed (SM). A GST packet is switched with constant, short switching delay, and without packet loss or reordering and is thus akin to transmitting the data over a circuit-switched network. An SM packet is switched using a packet-switching approach, in which some packet delay variation and packet loss are tolerated.
In the described hybrid approach, an optical packet-switch node in a wavelength-routed optical network separates incoming GST and SM packets using a polarisation beam splitter. GST packets are delayed using fibre delay lines for a fixed time corresponding to the longest SM packet and then follow wavelength paths through optical cross connects (OXCs) in the node. By contrast, SM packets are buffered, e.g. in electronic memory, and are switched according to their header information.
Incoming GST packets are delayed for the length of the longest SM packet to ensure that any active transmission of any SM packet can be completed, but the GST packets are then sent immediately after this delay. An SM packet is sent out from the node only when a sufficient number of wavelengths (greater than a predetermined minimum) are vacant. GST packets are thereby given priority over SM traffic.
In this known approach, incoming SM packets may be sorted into different electronic queues, where each queue contains SM packets whose lengths are in a range specific to that queue. The system requires fewer wavelengths to be free before an SM packet is sent out from a queue containing shorter packets, while more wavelengths are required to be free before an SM packet is sent out from a queue containing longer packets. Queues containing shorter packets may also be serviced with higher priority than queues containing longer packets. This approach can, to some extent, mitigate the risk of contention which is inherently higher when transmitting longer SM packets.
This known approach enables time-critical traffic, such as video-conference data, to be sent with guaranteed quality of service, while still making use of spare network capacity to send lower-priority traffic.
Nonetheless, the present inventor has come to realise that this approach can be improved upon.